1. Field of the Invention
The present invention relates to a method of establishing part-load operation in a gas turbine group.
2. Discussion of Background
When reduced power is required, it is known to effect the part load of gas turbine groups by means of a fuel control system. The turbine inlet temperature is reduced by reducing the fuel supply while the airflow remains constant. This intrinsically simple establishment of part-load operation is found to be insufficiently attractive with respect to efficiency, the efficiency falling, in fact, almost in direct proportion to the reduction in power.
In an attempt to keep this relatively high loss of efficiency within closer limits, it has already been proposed to undertake the reduction in power by means of an additional adjustment to the compressor guide vanes. In this control system, there is a smaller loss in thermal efficiency and the loss is of the order of value of up to two percentage points but it is only possible to reduce the mass flow, and therefore the power, down to approximately 80% with such a control system. The control range practiced in this manner, i.e. from 100% to 80%, is limited by the severe increase in the turbine outlet temperature, which is a consequence of a fall in pressure ratio at a turbine inlet temperature which is kept, as far as possible, at a constantly high level.
In a gas turbine group in which a high-pressure combustion chamber operates downstream of the compressor group and in which the hot gases are prepared for admission to a high-pressure turbine and in which the exhaust gases from the latter, which are at high temperature, subsequently flow through a so-called low-pressure combustion chamber before they are admitted to a low-pressure turbine, the requirements for establishing part-load operation have to be defined afresh, particularly where the low-pressure combustion chamber is designed for self-ignition.
Although it would be possible to decrease the power by closing the inlet guide vane row (where such a possibility is available at all), this would inevitably cause a reduction in the fuel quantity in the two combustion chambers of the gas turbine group. If the low-pressure combustion chamber is designed for self-ignition, the fuel quantities cannot be further reduced simultaneously because it would no longer be possible to maintain the temperature of the exhaust gases from the high-pressure turbine for subsequent self-ignition in the low-pressure combustion chamber.